Radio-frequency bridge having a delta input matching circuit



Oct. 11, 1966 c. WILSON 3,

RADIO-FREQUENCY BRIDGE HAVING A DELTA INPUT MATCHING CIRCUIT Filed Aug. 21, 1963 qLL-DB 0-5 1 .3 5 IO J0 300 500 1000 .7000

FREqwENcY- MCS INVENTOR. Cecil L. W'z'lson.

.BY 74 I Ja 51?? W F441 United States Patent 3,278,840 RADIO-FREQUEN CY BRIDGE HAVING A DELTA INPUT MATCHING CIRCUIT Cecil L. Wilson, Owings Mills, Md, assignor to the United States of America as represented by the Secretary of the Army Filed Aug. 21, 1963, Ser. No. 303,692 1 Claim. (Cl. 32457) The invention described herein may be manufactured and used by or for the Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to radio frequency bridges and more particularly to a new and novel wide band radio frequency bridge.

In the design, development and field testing of antennas some device is necessary to determine the impedance of the antenna. In particular, the device should show the degree of match between the transmission line feeding the antenna and the antenna itself. The degree of match is usually expressed in terms of voltage standing-wave ratio which if it is unity, the antenna is said to be matched to the transmission line and under such condition the antenna will absorb substantially all the energy flowing down the transmission line.

One disadvantage of commercially available voltage standing-wave ratio devices are that they are usable over but a limited range of frequencies. Therefore, several of these devices are required when it is necessary to match systems covering widely different frequencies. Another disadvantage of these commercial devices is that they require considerable radio frequency power, one to five watts, for acceptable operation and more power may be required if the use of an attenuator is necessary between the radio frequency generator and the voltage standingwave ratio device. An attenuator is usually necessary if a continuous measurement of voltage standing-wave ratio is made while the antenna impedance is adjusted to match the transmission line impedance. Another disadvantage of commercially available voltage standing-wave ratio devices is that the various impedances presented to the radio frequency generator causes a change in power output which results in a calibration error in the voltage standing-wave ratio device. This reaction on the radio frequency generator may also change the frequency of the generator during the antenna adjustment.

An object of the invention is a radio frequency bridge for measuring the voltage standing-wave ratio of antennas and transmission line loads over a range of frequencies extending from 0.5 to 2000 megacycles and wherein the input for the driving radio frequency generator and the output to the measured load are maintained at an impedance which substantially matches the generator and cables connecting these elements to the bridge.

Another object of the invention is a wide band standingwave ratio bridge requiring an input power in the order of milliwatts for the range of 0.5 to 2000 megacycles.

Another object of the invention is a wide band standing-wave ratio bridge having an electrical balance sufficiently great to allow accurate measurement of the voltage standing-wave ratio over the range of 0.5 to 2000 megacycles.

A still further object of the invention is a wide band radio frequency bridge wherein a delta input matching circuit improves the physical layout and thus the degree of electrical balance.

A better understanding of the invention may be had by referring to the following description of an exemplary embodiment of the invention taken in conjunction with the accompanying drawings, in which like references refer to like parts, in which:

FIGURE 1 is a schematic circuit diagram of the invention;

FIGURE 2 is a plan view of the physical embodiment of the invention with the upper half of the housing removed; and

FIGURE 3 is a graph showing the bridge null vs. frequency.

Referring now to FIGURE 1, the wide band radio frequency bridge comprises four arms Ra, Rb, Rs and Rz consisting of resistors 30, 31, 33 and antenna radiation resistance 32, respectively, and an arm Rc consisting of resistor 34 which bridges arms Ra and Rb at junctions 1 and 2, respectively. The opposite ends of arms Ra and Rb are connected at junction 3. Arms Rz and Rs each have one end connected to junctions 1 and 2 respectively and their opposite ends connected to junction 4. Arms Ra, Rb and Re form the delta input circuit of the bridge, each arm having the same value of resistance. Arm Rs contains the standard load resistor 33. Arm Rz is the arm which includes antenna 26 coupled by means of a coaxial cable 27 through the constant impedance connector 6. Constant impedance connector 8 provides means for coupling a radio frequency generator 29 through a coaxial cable 28 to the delta input circuit. A microammeter 9 calibrated to indicate the standing-wave ratio, has one terminal connected to one terminal of linearizing resistor 10 and its other terminal connected to one terminal linearizing resistor 11 through the two terminal meter jack 20. The opposite terminals of resistors 10 and 11 are coupled to junctions 1 and 2 through capacitors 12 and 13 respectively. The diode rectifier 15 is connected in parallel with the microammeter at the-aforementioned opposite terminals of resistors 10 and 11. Serially connected coaxial by-pass capacitors 16 and 17 are shunted across the microarnmeter and grounded at their midpoint.

FIGURE 2 illustrates a physical embodiment of the invention wherein the rectangular aluminum housing 18 consists of two identical members comprising the top and bottom of the housing which may be fastened together by any well known means. Since these members are identical, only the bottom member 18a is shown. The housing is provided with a centrally located longitudinal bore 19 in which the circuit elements of the bridge are positioned. The end 21 of the housing has affixed thereto in communication with bore 19 a constant impedance coupler 8 for coupling a radio frequency generator to junction 3 of the delta input circuit. Bores 22 and 23 are formed in the sidewalls of the housing at a right angle to the bore 19 and in alignment with member Rc of the delta input circuit. Positioned in bore 22 is a constant impedance connector 6 for coupling antenna 26 through a coaxial cable 27 to junction 1 of the delta input circuit. Positioned in bore 23 is a constant impedance connector 14 which houses the standard load resistor 33 of arm Rs and connects it to junction 2 of the delta input circuit and junction 4. The bridge circuit is completed through the casing of constant impedance connectors 6, 8 and 14 which are secured to the aluminum housing 18. Coaxial by-pass capacitors 16 and 17 of the voltmeter circuit are positioned in bores 24 and 25 in the side walls of the housing, respectively, adjacent the meter jack 20 which is affixed in the bore 19 at end 25 of housing 18. Coaxial capacitors 16 and 17 each have one terminal grounded by means of the housing. The other terminal of each of said capacitors is connected to the respective terminals of meter jack 20. It is to be noted that the circuit elements of the delta input circuit and the circuit elements of the voltmeter circuit are symmetrically arranged in the bore 19 whereby a great degree of electrical balance is obtained.

In operation a radio frequency generator 29 is coupled to connector 8 by means of a coaxial cable 28 and the microammeter connected to meter jack 20. The standard load resistor 33 is coupled to the bridge by means of constant impedance connector 14. The radio frequency generator is adjusted until the microammeter reads full scale whereby a reference input power is obtained. The antenna 26 to be measured is then connected by means of a coaxial cable 27 to connector 6. The reading of the microammeter indicates the voltage standing-wave ratio. The bridge has a balance greater than 30 db from 0.5 to 1000 megacycles and a balance greater than 25 db over the range of 1000 to 2000 megacycles.

It Will be apparent that the embodiment shown is only exemplary and that various modifications can be made within the scope of the invention'as defined in the ap pended claim.

I claim: 7

A standing wave ratio meter bridge comprising a metal housing provided with a longitudinal bore therethrough, a first coaxial constant impedance connector afiixed to said housing at one end of said longitudinal bore for coupling a radio frequency generator to said bridge, a symmetrical electrical jack afiixed to said housing at the opposite end of said longitudinal bore for coupling a current indicator means to the output of said bridge, said jack provided with two terminals projecting into said longitudinal bore and equally spaced from the longitudinal axis thereof, a first transverse bore adjacent said jack in said housing and extending therethrough, the longitudinal axis of said first transverse bore intercepting the longitudinal axis of said longitudinal bore, a first coaxial capacitor fixed in one end of said first transverse bore with the center terminal thereof connected to one of said spaced terminals, a second coaxial capacitor fixed in the opposite end of said first transverse bore with the center terminal thereof connected to the other terminal of said spaced terminals, a second transverse bore adjacent said first coaxial constant impedance connector in said housing and extending therethrough, the longitudinal axis of said second transverse bore intercepting the longitudinal axis of said longitudinal bore, a second coaxial constant impedance connector affixed to said housing at one end of said second transverse bore for coupling an element to be measured into an arm of said bridge, a third coaxial constant impedance connector affixed to said housing at the opposite end of said second transverse bore, a resistor provided with two leads comprising the standard arm of said bridge housed in said third coaxial constant impedance connector having one of said leads connected to the center terminal of said connector, the other of said leads connected to the housing of said connector, a delta input circuit for matching the impedance of said generator to the impedance of the eleand second resistors being connected to the center terminal of said coaxial constant impedance connector, said first resistor directed diagonally from said longitudinal axis and positioned midway between the center terminals of said first and second coaxial constant impedance connectors, the other lead of said first resistor being connected to the center terminal of said second coaxial constant impedance connector, said second resistor directed diagonally in the same plane but in a direction opposite to that of said first resistor from said longitudinal axis and positioned midway between the center terminals of said first and third coaxial constant impedance connectors, said third resistor having one lead connected to the center terminal of said second coaxial constant impedance connector and its other lead connected to the center terminal of said third coaxial constant impedance connector and positioned midway therebetween, first and second linearizing each having two leads spaced parallel to each other and equally from the wall of said longitudinal bore in the same plane as the'delta input circuit, first and second coupling capacitors each having two leads and equally spaced from the wall of said longitudinal bore, one lead of said first linearizing resistor connected to said one of said spaced terminals, said first coupling capacitor coupling the other lead of said first linearizing resistor with said one lead of said third resistor, one lead of said second linearizing resistor connected to said other terminal of said spaced terminals, said second coupling capacitor coupling the other lead of said second ment to be measured consisting of first and second resistors comprising the ratio arms of said bridge and a third resistor, each of said first, second and third resistors provided with two leads, one lead of each of said first linearizing resistor with said other lead of said third resistor, said coupling capacitors being spaced parallel to each other in the same plane as said delta circuit and equally spaced from the wall of said longitudinal bore, and a diode rectifier provided with two leads, said diode rectifier positioned in the plane of the delta input circuit intermediate said other leads of said first and second linearizing resistors and connected therebetween to said other leads of said first and second linearizing resistors.

References Cited by the Examiner OTHER REFERENCES Hubbard et al., Journal of Scientific Instruments, "Instantaneous Measurement of a Varying Frequency, vol. 29, November 1952, pp. 366470.

WALTER L. CARLSON, Primary Examiner. E. E. KUBASIEWICZ, Assistant Examiner. 

